I've done some quick research. As Darkseeker has mentioned, there are a number of reasons for Avast not being able to scan certain files such as archives, password-protected files etc. This is normal when doing malware scans with a variety of different programs....just some don't report that kind of info. In short, there is no reason to believe that you have a problem or that your PC is infected. Avast is just letting you know that some files were not accessible to scan them. If you want to take extra measures to reassure yourself, do what Darkseeker suggested and run Malwarebytes free edition on your PC.